Iron oxide reduction



Aug. 18, 1959 P. c. KEITH ETAL 2,900,246

IRON OXIDE REDUCTION Filed Jan. 14, 1953 25 I @1 z; i 27 A INVENTORS PERC/VAL C. KEITH HERMAN N. WoEBCKE HGENT nited States IRON OXIDE REDUCTION Application January 14, 1953, Serial No. 331,142

.11 Claims. (Cl. 75-26) This invention relates to the reduction of iron oxide in finely divided form by intimate contact and reaction with hydrogen.

The voluminous prior art on the reduction of iron oxide includes numerous different approaches to the solution of this basic operation of the steel industry. The various proposals embrace treating the iron oxide in particle sizes ranging from fine powders to substantial briquets, using reducing agents that are solid and/or gaseous, and maintaining the iron oxide, while undergoing reduction, in a stationary bed, in a downwardly moving column, in a dispersed state in a gas or in a highly fluidized dense phase. In spite of the attention which has been given to all the chemical and physical factors which influence the reduction of iron oxide, shortcomings are well recognized in each of the processes that have been proposed to date or that are currently in commercial operation. Thus, some processes require that the iron oxide be pelleted or briquetted and this requirement obviously increases the cost of producing iron. Coke and like solid reducing agents are necessary to many processes but the supplies of such materials are limited and, in any event, are cumbersome from a handling point of view. Processes depending on the use of stationary beds or moving columns inherently have the disadvantage of pellets or briquets because pellets or briquets are essential to forming a mass containing interstices for the passage of reducing and reaction gases therethrough. Reducing systems employing a dispersion of powdered iron oxide in a suspending gas call for large reactors and costly gas-solid separators. All attempts to operate with the iron oxide in a highly fluidized condition have failed to become sufliciently attractive for commercial adoption because a fluidized mass is ofuniform composition throughout whereas a composition gradient is generally desired. Some workers have recognized this drawback of fluidized masses and have proposed to overcome itby using a complicated series of vessels or a series of hearths in one vessel, each containing a fluidized mass that in compositionis more reduced than the preceding mass andlessreduced than the succeeding mass.

A principal object of this invention is to reduce iron oxide in finely divided form without encountering the prior disadvantages ofv using a gaseous dispersion of the iron oxide or an intricate series of fluidized beds thereof.

Another important object is to effect'the reduction of iron oxide in the absence of solid reducing agents.

A further object is to reduce iron oxide while avoiding any substantial agglomeration or consolidation of the fine particles undergoing reduction.

These and additional objects and advantages of the invention will beapparent 'from' the description which follows. i Y i n In accordance with this invention, finely divided iron 2 oxide, usuallyan ore concentrate obtained by flotation, magnetic separation or a like concentrating process, is intimately contacted and reacted with high-purity hydrogen at temperatures in the range of about 700 to 1000 F., preferably about 850 to 950 F., and at pressures in the range of about 200 to 600 p.s.i.g. (pounds per square inch gauge), preferably about 350 to 450 p.s.i.g. For the purposes of this invention, high-purity hydrogen means hydrogen containing only small amounts of such gases as carbon monoxide and dioxide, methane, nitrogen and water vapor; more specifical y, the high-purity hydrogen used in the process of this invention generally has a composite average molecular weight in the range of about 3 to 7.5, preferably less than about 5.5. Since hydrogen has a molecular weight of 2 and the aforesaid admixed gases have molecular weights varying from'16 to 44, it is obvious that the quantities of the admixed gases must be kept small to hold the composite average molecular Weight below the upper limit of 7.5. In addition to the composite average molecular weight limitation, the high-purity hydrogen charged into the reactor is further restricted in that its moisture content is initially not more than'is indicated in the following table:

Reaction Maximum 1120 Vapor Content; Based on H: Content Tempelr ature,

0.5% by volume 700 1.0% by volume 800 1 5% byynlnme 850 2.0% by volume 900 2.5% by volume 950 3.0% by volume 4 1, 000

.In short, the high-purity hydrogen supplied to a reactor this invention, the iron oxide is maintained as a layer or bed, usually not exceeding about 5 feet in depth, while the pressurized high-purity hydrogen is passed upwardly therethrough at a rate sufficient to mobilize the particles without causing the excessive agitation characteristic of fluidization. While fluidization has been often likened in appearance to a mass of boiling water, the slow, quasiviscous movements of the particles undergoing mobilization pursuant to this invention may be more closely likened in appearance to a mass of molten lava. For the usual particle sizes and densities of the iron oxide treated by this invention, the reducing gas will generally have a superficial linear velocity of the order of 0.5 foot per second while contacting the mass of iron oxide.

The. intimacy and consequent effectiveness of the contact between the iron oxide particles and the high purity hydrogen is ensured by partitioning the mobilized layer or bed with substantially vertical surfaces which are immersed therein and are spaced from one another in any desired manner satisfying the condition that the particles in the bed which are farthest from any such surface shall not be less than about lsinch and not more than about 6 inches away from any such surface. For simplicity,

hereinafter, this condition shall be referred to as an equivalent radius in the range of about 1 to 6 inches; an equivalent radius in the range of 2 to '3 inches is preferred. When the process of this invention is conducted batchwise, the partitioning surfaces may be imperforate, but where continuous operation is desired, the surfaces will have small openings or slits to permit the migration of particles from one partitioned portion of the iron oxide layer or bed to a contiguous portion, and so on, the oxide particles thus progressing through a series of partitioned but communicating portions of the mobilized bed. In continuous operation, fresh iron oxide particles are charged to one end of the series of partitioned portions of the mobilized bed and reduced iron particles are withdrawn from the other end of the series.

A highly desirable embodiment of the process of the invention involves the reduction of iron oxide in a shallow layer by treatment with high-purity hydrogen of very limited moisture content at an elevated pressure. The specific manner in which the process variables have been integrated is apparently responsible for the success of the invention. The mobilized layer or bed of iron oxide is maintained shallow, i.e., usually not more than about 5 feet deep and generally at least 1 foot deep, because with the intimate contact achieved through particulate mobilization and the rate of reaction obtained by using hydrogen at elevated pressure, the hydrogen passing through the mobilized bed will have picked up such an amount of vaporized water of reaction by the time the gas has traversed the depth of the bed that the reducing gas is no longer satisfactorily effective for reducing iron oxide.

It is also noteworthy that by operating at pressures in the range of 200 to 600 p.s.i.g. the moisture content of the reaction gases withdrawn from the iron oxide bed is simply reduced to less than about 0.4% by volume by cooling these gases to a temperature of 100 F. or lower with water at the temperatures available in most localities. In fact, the withdrawn reaction gases may be water-washed to effect this cooling-moisture removal and simultaneously to separate fine particles that may become entrained in the gases withdrawn from the mobilized bed undergoing reduction. Reaction gases from which water of reaction has been thus removed are largely recycled for further contact with the iron oxide without any intervening treatment to change the composition of these gases. Indeed, another distinguishing aspect of the process of this invention is the high ratio of recycle gases to fresh feed or make-up gases that are passed into contact with the iron oxide, this ratio generally exceeding 30 volumes of recycle gases to 1 volume of make-up gases and frequently being in the range of 50:1 to 70:1. Regardless of the quantity of the reaction gases that may be recycled for further contact with iron oxide, after the moisture content of these gases has been decreased, preferably to not more than 0.4% by volume, it will be re- '7 membered that the mixture of recycle and make-up gases must have a composite average molecular weight of not more than 7.5. Only that proportion of the reaction gases is vented or discarded as will prevent the content of such gases as methane and nitrogen from building up to an extent that the mixed recycle and make-up gases introduced into the mobilized iron oxide bed have a composite average molecular weight exceeding 7.5. Commercially available processes for the production of hydrogen yield a gas generally containing better than 90% by volume of hydrogen and not more than about 3% v by volume of the carbon oxides, principally carbon monoxide. Such a gas-is advantageously utilized as the fresh feed or make-up gas.

Reference is now made tothe drawings of which:

Figure 1 is a diagrammatic fiowsheet of one illustrative embodiment of the invention; and

Figure 2 is a horizontal section of the reducing reactor or Figure 1, taken along the line 22.

Finely divided iron oxide entering by way of line 10 is passed downwardly through preheat tower 11 against upflowing hot reaction gases passing from reducing reactor 12 through line 13. Reactor 12 is desirably a cylindrical vessel disposed with its axis horizontally and having a perforated or foraminous plate 14 to support a shallow bed of the oxide. High-purity hydrogen introduced through line 15 fills the distributing space 16 and thence passes uniformly up through all of plate 14 to mobilize the layer of iron oxide thereon. Vertical surfaces 17 partition the oxide layer into a plurality of small portions, each having an equivalent radius in the range of about 1 to 6 inches, preferably about 2 to 3 inches. The partitioned portions of the iron oxide layer are arranged in a communicating series having flow access from one portion to the next through perforations or slits 18 in the vertical surfaces 17. As shown in Figure 2, the preheated iron oxide leaving tower 11 through line 19 enters the first partitioned portion of the mobilized bed in reactor 12 and progresses in zigzag fashion through the series of partitioned portions by way of the perforations 18 in vertical surfaces \17 until it reaches the final partitioned portion in a substantially reduced state whence the reduced material is withdrawn through line 20. The reaction gases leaving dome 21 of reactor 12 are in part passed through line 13 as aforesaid and in part through line 22 and heat exchanger 23 into wash tower 24. Scrubbing water is supplied by line 25 and drained through line 26. The cooled, washed gases leave tower 24 through line 27. The portion of the reaction gases which flows out of tower 11 by way of line 28 may be added to the other portion entering wash tower 24. The total reaction gases minus the water of reaction eliminated in wash tower 24 and minus the small proportion of the gases discarded through valved vent line 29 to prevent the build-up of the content of gases like nitrogen and methane are drawn into compressor 30 which raises the pressure sufiiciently to compensate for the pressure losses the reaction gases had in passing through the plant. The recompressed gases are recycled to reactor 12 after passing through line 31, heat exchanger 23, and heater 32. Fresh hydrogen is added as make-up to the recycled gases by way of line 33.

In a specific example, finely divided hematite is charged to preheat tower 1 1 at the rate of 800 tons per day whence it discharges at a temperature approaching 800 F. and then enters reactor 12. High-purity hydrogen (composite average molecular weight of 5.4) with a water vapor content of about 0.3% by volume, based on the volume of hydrogen in said high-purity hydrogen, is charged through line 15 at a pressure of 400 psig. and with sufficient preheat to maintain the layer of solids on plate 14 at a temperature of about 850 F. The gas passes up through the layer of solids with a superficial linear velocity of about 0.4 foot per second thereby effecting mobilization and reduction of the oxide particles. The layer is about 3 feet deep and the reaction gases emerge from the top thereof with a moisture content of about 2% by volume. After passing through wash tower 24, these gases at a temperature of 100 F. contain only about 0.25% by volume of water vapor; a small proportion of these gases is vented through line 29. Fresh hydrogen (analysis on volume basis: 94.7% H 3% CO, 0.1% CO 0.4% CH 0.3% H 0, 1.5% N A, etc.) is added to the recycled reaction gases at the rate of nearly 15 MM s.c.f.d. (million standard cubic feet per day). The volume ratio of recycled gases to fresh make-up gas is approximately 50:1. The partitioned portions of the mobilized bed in reactor 12 have an equivalent radius of 2 inches. The material withdrawn through line 20, on analysis, shows that more than by weight of the hematite has been completely reduced to iron.

'In view of the various modifications of the invention which will occur to those skilled in the art upon consideration of the foregoing disclosure without departing'from the spirit or scope thereof, only such limitations should be imposed as 'are'indicated by the appended claims.

What is claimed is:

1. The process' of reducing i'on oxide which comprises passing high-purity hydrogenupwardly through a bed of finely'divided iron oxide at a rate to cause mobility of said bed, the horizontal extent of said mobilized-bed being interrupted by closely spaced; substantially vertical surfaces so that the particles in said mobilized bed which are farthest from any vertical surface are not less than about 1 inch and not more than about '6 inches away from any vertical surface, and maintaining said mobilized bed at a temperature in the range of about 700 to 1000 F. and at a pressure of about 200 to 600 p.s.i.g. while said high-purity hydrogen is passing therethrough, said highpurity hydrogen containing small amounts of carbon oxides, methane, nitrogen and water vapor and initially having a composite average molecular weight not exceeding about 7.5 and a maximum water vapor content related to the reaction temperature such that said water vapor content does not exceed about 0.5% by volume of the hydrogen content of said high-purity hydrogen at a reaction temperature of 700 F. and does not exceed about 3 by volume of the hydrogen content of said high-purity hydrogen at a reaction temperature of 1000 F.

2. The process of claim 1 wherein the reaction gases emerging from the upper surface of said mobilized bed are, without substantial decrease of pressure, waterwashed to a temperature not exceeding about 100 F., and the predominant portion of the washed gases is recycled to provide at least 30 volumes of each 31 volumes of said high-purity hydrogen passed upwardly through said mobilized bed.

3. The process of reducing iron oxide which comprises passing high-purity hydrogen upwardly through a bed of finely divided iron oxide at a rate to cause mobility of said bed, the horizontal extent of said mobilized bed being interrupted by closely spaced, substantially vertical surfaces so that the particles in said mobilized bed which are farthest from any vertical surface are not less than about 2 inches and not more than about 3 inches away from any vertical surface, and maintaining said mobilized bed at a temperature in the range of about 850 to 950 F. and at a pressure of about 350 to 450 p.s.i.g. while said high-purity hydrogen is passing therethrough, said high-purity hydrogen containing small amounts of carbon oxides, methane, nitrogen and water vapor and initially having a composite average molecular weight not exceeding about 5.5 and a maximum water vapor content not exceeding about 0.4% by volume of the hydrogen content of said high-purity hydrogen.

4. The process of claim 3 wherein the reaction gases emerging from the upper surface of said mobilized bed are, without substantial decrease of pressure, waterwashed to a temperature not exceeding about 100 F., and the predominant portion of the washed gases is recycled to provide at least 50 volumes of each 51 volumes of said high-purity hydrogen passed upwardly through said mobilized bed. 7

5. The process of claim 1 wherein at least a portion of the reaction gases emerging from the upper surface of said mobilized bed is passed in countercurrent heat exchange contact with fresh finely divided iron oxide to preheat the same and the thus preheated iron oxide is then reduced as already specified.

6. The process of claim 1 wherein the reaction gases emerging from the upper surface of said mobilized bed are, without substantial decrease of pressure, waterwashed to a temperature not exceeding about 100 F., and the predominant portion of the washed gases is recycled, after being heated to a temperature above the temperature at which said mobilized bed is maintained, as the predominant portion of said high-purity hydrogen passed upwardly through said mobilized bed.

- 7. The process of reducing iron oxide which comprises passing high-purity hydrogen upwardly through a bed of finely divided iron oxide at a rate to cause mobility of said bed,'the horizontal extent 'of said mobilized bed being interrupted by closely spaced, substantially vertical surfaces so that the particles in said mobilized bed which are farthestfrom any vertical surface are on the order of 3 inches' away from any vertical surface, and maintaining said mobilized bed at a temperature in the range of about 700 to 1000 F. and at a pressure of about 200 to 600 p.s.i.g. while/said high-purity hydrogen is passing therethrough, said high-purity hydrogen containing small amounts of carbon oxides, methane, nitrogen and water vapor and initially having a composite average molecular weight not exceeding about 7.5 and a maximum water vapor content related to the reaction temperature such that said Water vapor content does not exceed about 0.5% by volume of the hydrogen content of said high-purity hydrogen at a reaction temperature of 700 F. and does not exceed about 3% by.volume of the hydrogen content of said high-purity hydrogen at a reaction temperature of 1000 F., withdrawing the resulting gases from contact with said mobilized bed, cooling the withdrawn reaction gases without substantial decrease of pressure to a temperature not exceeding about F. thereby condensing out water vapor, separating the condensed water from the cooled gases, and recycling the thus =dehumidified gases, after being heated to a temperature above the temperature at which said mobilized bed is maintained, as the predominant portion of said high-purity hydrogen passed upwardly through said mobilized bed.

8. The process of claim 7 wherein the cooling of the withdrawn reaction gases without substantial decrease of pressure to a temperature not exceeding about 100 F. comprises washing said withdrawn gases with water.

9. The process of reducing iron oxide which comprises passing high-purity hydrogen upwardly through a bed of finely divided iron oxide at a rate to cause mobility of said bed, the horizontal extent of said mobilized bed being interrupted by closely spaced, substantially vertical surfaces so that the particles in said mobilized bed which are farthest from any vertical surface are on the order of 3 inches away from any vertical surface, and maintaining said mobilized bed at a temperature in the range of about 700 to 950 F. and at a pressure of about 350 to 600 p.s.i.g. while said high-purity hydrogen is passing therethrough, said high-purity hydrogen containing small amounts of carbon oxides, methane, nitrogen and Water vapor and initially having a composite average molecular weight not exceeding about 7.5 and a maximum water vapor content not exceeding about 0.5% by volume of the hydrogen content of said high-purity hydrogen, withdrawing the resulting gases from contact with said mobilized bed, cooling the withdrawn reaction gases without substantial decrease of pressure to a temperature not exceeding about 100 F. thereby condensing out water vapor, separating the condensed water from the cooled gases, and recycling the thus dehumidified gases, after beingheated to a temperature above the temperature at which said mobilized bed is maintained, as the predominant portion of said high-purity hydrogen passed upwardly through said mobilized bed.

10. The process of claim 9 wherein the cooling of the withdrawn reaction gases without substantial decrease of pressure to a temperature not exceeding about 100 F. comprises washing said withdrawn gases with Water.

11. The process of claim 3 wherein the reaction gases emerging from the upper surface of said mobilized bed are, without substantial decrease of pressure, waterwashed to a temperature not exceeding about 100 F., and the predominant portion of the washed gases is re cycled, after being heated to a temperature above the temperature at which said mobilized bed is maintained,

'7 8 as the predominant portion of said high-purity hydrogen Udy et a1'.: The Low-Temperature Gaseous Reduction passed upwardly through said mobilized bed. of Magnetite, Transactions American Institute of Mining and Metallurgical Engineers; vol. 154; pp. 162-.-181, 1943.

References Cited in the file of this pat ent sphect, Jr et ThelLowTempemture' Gaseous UNITEDSTATES PATENTS 5 duction of Magnetite OreitoSponge Iron, Transactions 2,389,133 Brassert et a1. Nov. 20, 1945 American Institute, of ;Mining and Metallurgical En- 2,441,594 Ramseye May g, 9 8 .Y .1 7; PP- 6- 2,671,765 McGrath et aL Man 9, 1954 Tennenbaum et al.: Dll'CCt Reduction of Iron Ore Using the Fluidized Solids Technique. Reprint of paper OTHER REFERENCES 10 to be read before General Meeting of American Iron Transactions of the American Institute of Mining and and Steel Institute, at New York, May 23-24, 1951. Metallurgical Engineers, vol. 135, pages 6771, 1939. 

1. THE PROCESS OF REDUCING IRON OXIDE WHICH COMPRISES PASSING HIGH-PURITY HYDROGEN UPWARDLY THROUGH A BED OF FINELY DIVIDED IRON OXIDE AT A RATE TO CAUSE MOBILITY OF SAID BED, THE HORIZONTAL EXTENDED OF SAID MOBILIZED BED BEING INTERRUPTED BY CLOSELY SPACED, SUBSTANTIALLY VERTICAL SURFACES SO THAT THE PARTICLES IN SAID MOBILIZED BED WHICH ARE FARTHEST FROM ANY VERTICAL SURFACE ARE NOT LESS THAN ABOUT 1 INCH AND NOT MORE THAN ABOUT 6 INCHES AWAY FROM ANY VERTICAL SURFACE, AND MAINTAINING SAID MOBILIZED BED AT A TEMPERATURE IN THE RANGE OF ABOUT 700* TO 1000*F. AND AT A PRESSURE OF ABOUT 200 TO 600 P.S.I.G. WHILE SAID HIGH-PURITY HYDROGEN IS PASSING THROUGHOUT, SAID HIGHPURITY HYDROGEN CONTAINIING SMALL AMOUNTS OF CARBON OXIDES, METHANE, NITROGEN AND WATER VAPOR AND INITIALLY HAVING A COMPOSITE AVERAGE MOLECULAR WEIGHT NOT EXCEEDING ABOUT 7.5 AND A MIXIMUM WATER VAPOR CONTENT RELATED TO THE REACTION TEMPERATURE SUCH THAT SAID WATER VAPOR CONTENT DOES NOT EXCEED ABOUT 0.5% BY VOLUME OF THE HYDROGEN CONTENT OF SAID HIGH-PURITY HYDROGEN AT A REACTION TEMPERATURE OF 700*F. AND DOES NOT EXCEED ABOUT 3% BY VOLUME OF THE HYDROGEN CONTENT OF SAID HIGH-PURITY HYDROGEN AT A REACTION TEMPERATURE OF 1000*F. 